generating permutations

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Sorry, I am very frustrated right now. I have spent the last six hours trying to come up with a simple perl program to list all permutations for the numbers from one to five (the use of cpan modules is not allowed for this exercise).

The well-known algorithm and code snippets in other languages can be found here:

http://stackoverflow.com/questions/9148543/program-to-print-permutations-of-given-elements

One python implementation is very brief and almost understandable (though my python knowledge is even poorer than my perl knowledge):

def permute(in_str, prefix=""):
    if not len(in_str) :
        print(prefix)
    else:        
        for i in range(0, len(in_str)):

            permute(in_str[:i] + in_str[i + 1:], prefix + in_str[i])
permute('12345')
[download]

The biggest problem is (even after reading man perlsub for over an hour), I just don't understand the calling syntax. My attempt is looking something like this (which generates a huge number of syntax errors and won't run).

#!/usr/bin/perl -w
use strict;

my @array = 1..5;

while (permute @array)

sub permute
    {
    my @temp;
    while ( $#temp < $#_)

        {
        my $i;
        for ($i = $#temp; $i < $#_; $i++)

            {
            swap @_[$i] @_[$#temp];
            permute;
            swap @_[$i] @_[$#temp];
            }
        }

    print $_;
    }

sub swap
    {
    my $k;
    my $l;
    my  $temp;
    $temp = $k;
    $k = $l;
    $l = $temp;
    }
[download]

Can someone guide me in the right direction. Clever techniques are not important. What is important is that I understand what is going on.

Thank you (very much) in advance.

Comment on generating permutations Select or Download Code

Replies are listed 'Best First'.
Re: generating permutations by toolic (Bishop) on Mar 29, 2017 at 17:33 UTC
Start smaller. Here is one basic way to create and use the swap sub: `use warnings; use strict; my @swaps = swap(2, 3); print "@swaps\n"; sub swap { my $k = shift; # 1st arg my $l = shift; # 2nd arg return $l, $k; # List of 2 args swapped } __END__ Prints: 3 2` [download] Writing subroutines shift Use parentheses after the sub name when calling. Separate args with commas. Basic debugging checklist	[reply] [d/l]
Re: generating permutations by BrowserUk (Patriarch) on Mar 29, 2017 at 19:00 UTC
Here's a brute force conversion of the python code you posted: `#! perl -slw use strict; sub permute { my( $in, $pre ) = ( @_, '' ); if( not length $in ) { print $pre; } else { for my $i ( 0 .. length( $in ) - 1 ) { permute( substr( $in, 0, $i ) . substr( $in, $i+1 ), $pre . substr( $in, $i, 1 ) ); } } } permute( '12345' );;` [download] Ask if anything is unclear. With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity. In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l]
Re^2: generating permutations by Anonymous Monk on Mar 29, 2017 at 20:36 UTC
Thanks, again. With a few extra print statements, I am able to see exactly what it is doing. A quick (I hope) follow up. How would I do a more general list - say all the permutations of (apple, banana, orange), or numbers that go into double digits?	[reply]
Re^3: generating permutations by BrowserUk (Patriarch) on Mar 29, 2017 at 21:21 UTC
Try this. It's a re-casting of the Python algorithm to operate on arrays: `#! perl -slw use strict; sub permute3 { my( $in, $pre ) = ( @_, [] ); return print join '-', @$pre unless @$in; permute3( [ @{ $in }[ 0 .. $_-1, $_+1 .. $#$in ] ], [ @$pre, $in->[ $_ ] ] ) for 0 .. $#$in; } permute3( [ qw[ apple banana orange ] ] );; __END__ C:\test>junk36 apple-banana-orange apple-orange-banana banana-apple-orange banana-orange-apple orange-apple-banana orange-banana-apple` [download] With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity. In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l]
Re^3: generating permutations by tybalt89 (Monsignor) on Mar 31, 2017 at 22:35 UTC
Here's a version that uses a separate array to mark when a item is used instead of building and destroying anon arrays. `#!/usr/bin/perl # http://perlmonks.org/?node_id=1186402 use strict; use warnings; my @items = qw( apple banana orange ); my @used; sub permute { @_ == @items and return print "@_\n"; $used[$_]++ \|\| permute(@_, $items[$_]), $used[$_]-- for 0 .. $#items +; } permute();` [download]	[reply] [d/l]
Re: generating permutations by tybalt89 (Monsignor) on Mar 29, 2017 at 17:42 UTC
`#!/usr/bin/perl -l # http://perlmonks.org/?node_id=1186402 use strict; use warnings; $_ = 12345; 1 while print, s/.\K # find the last (.) # digit such that (.) # there is a later (latest) (.)(??{$1 > $3 and 'x'}) # digit greater than it (.*) # and get rest # swap those two digits ( $1 & $3 ) # then reverse everything after the first swapped digit / $3 . reverse $2.$1.$4 /xe` [download]	[reply] [d/l]


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